The invention concerns a process for the manufacture of hydroquinones and more particularly 2, 3, 5 trimethylhydroquinones through the catalytic reduction of the corresponding quinone with hydrogen at low pressures and temperatures.
German patent application publication 1956381 teaches forming hydroquinone derivatives, for example, 2, 3, 5 trimethylhydroquinone through catalytic hydrogenation, using copper chromite catalytic agents, of the corresponding quinone in solution at elevated pressures (80-150 atmospheres) and temperatures (50.degree.-150.degree.C, or higher). As a solvent one can use alcohol (for example, methanol, ethanol, etc.), ether (for example, dioxane) or water. The processing at elevated pressures and temperatures requires complex equipment. The process has the further drawback that the end products are not pure and must be purified through recrystallization in the presence of a stabilizer.
German patent 683908 teaches catalytic hydrogenation of trimethyl-p-quinone in the presence of an organic solvent and a palladium catalyst, to trimethylhydroquinone. Solvents that can be used are ether, glacial acetic acid, dilute acetic acid, toluol alcohol and alcohol, apparently ethanol. In working in accordance with the teaching of this patent with methanol or ethanol one obtains in part very impure end products. These show strong coloration even if a filtrate is separated from the catalyst in the absence of atmospheric oxygen, and evaporated as it is passed through carbon dioxide, as called for in the patent.
In particular during hydrogenation in reaction vessels made of stainless steel one obtains exceptionally strongly colored trimethylquinone caused by the formation of black quinhydrone which makes it impossible to use the end product for further processing as for example in the manufacture of vitamin E. If one uses one of the other solvents called for in patent 683908, then the trimethylhydroquinone resulting from the hydrogenation precipitates and the catalyst becomes encrusted and enclosed and therefore ineffective shortly after starting the process. The same is true in the utilization of aromatic hydrocarbons, for example toluol, aliphatic or cycloaliphatic hydrocarbons alone or mixed with each other, for example, in hexane ligroine and cyclohexane in higher alcohols, for example n-heptanol. Under these circumstances, the high boiling solvents are separated from the end product only with the greatest difficulty.
German patent application publication 1940386 carries out the hydrogenation of trimethyl-p-quinone with hydrogen to trimethylhydroquinone in the presence of an alcohol of 3-5 carbon atoms and at a temperature of 60.degree.-180.degree.C. In this way, the autooxidation, as indicated by the coloration of the end product, is said to be greatly reduced, particularly if one uses branched alcohols of the described type as solvents. However the drawbacks of this process are that the hydrogenation, in contrast to patent 683908, must be carried out at elevated temperatures and that on the basis of the relatively high solubility of the end product in the alcohol used, the starting brine must be used several times. Thereby the impurities are continually enriched, which eventually can lead to a reduction of quality of the trimethylhydroquinone through cocrystallization of by-products, which should particularly be avoided.
It is therefore an important object of the present invention to provide an economical process which will not be characterized by the cited drawbacks and which will lead in particular to very pure and light hydroquinones.